1. Field of the Invention
The present invention relates to a graphite precipitated hot-rolled steel plate having excellent bending workability and hardenability and a manufacturing method therefor. More particularly, the present invention relates to a graphite precipitated hot-rolled steel plate (hereinafter called a "graphite steel plate") having excellent bending workability and enabling satisfactory strength and wear resistance which is obtained by hardening performed after machining has been performed and a manufacturing method therefor.
2. Related Art Statement
A major portion of mechanical parts are given strength and wear resistance by a heat treatment performed after the forming work. In order to meet the strength and the wear resistance requirements realized after completion of the heat treatment, high carbon steel has been used. However, since much of the production of steel has been changed from hot forming to cold forming in recent years in order to improve the manufacturing yield and to suppress decarbonization, there has been a need to improve the cold workability of the conventional high carbon steel. To this end, inventors of the present invention have found that the workability can be improved by making the microstructure of high carbon steel to have a graphite precipitated in the ferrite. The inventors also found that it is necessary to fine down the graphite present in the ferrite in order to improve the hardenability of the steel. They also found that an addition of B is significantly effective and beneficial. Therefore, a technology has been disclosed in Japanese Patent Laid-Open No. 2-107742 and Japanese Patent Laid-Open No. 2-111842, which enables a hot-rolled steel plate, which has excellent bending workability and heat hardenability, to be manufactured by subjecting the steel containing B to an annealing process after the hot rolling step.
The above-described disclosures teach how graphite particles can be significantly finely precipitated in comparison to the conventional technology. These disclosures establish that cold workability superior to that obtainable from conventional carbide-spheroidizing annealing (steel having spheroidal carbide) can be realized while having hardenability corresponding to the quantity of carbon.
However, steel of the type manufactured by a method according to the above-described prior art encounters a problem in a case where it is formed as a hot-rolled steel plate, and in particular, in a case where it is subjected to bending work. The reason for this was researched, and it was found that the distance between graphite particles is too short although the graphite particles are fined down or small. As a result, the graphite particles act as if they are large deposits and thereby they serve as the initiation point or the propagation route of the cracks which will take place at the time of the bending work.